GPRS (General Packet Radio Service) is a service that provides wireless packet data access to mobile GSM (Global System for Mobile Telecommunications) users. GPRS provides the first implementation of a packet switching technology within GSM, which is, itself, a circuit switched technology. GPRS is also an important precursor for 3G (3rd Generation Mobile Technology) as it implements the packet switched core required for UMTS (Universal Mobile Telecommunications System) networks. Data rates in the order of 115 kb/s can be supported.
One of the main features of GPRS is that it reserves radio resources only when there is such a need and that these radio resources are shared by all mobile subscribers (MS's) in a cell, where a cell is the geographical area covered by a cellular base station.
Therefore, as is the case for all packet data services, effective resource utilization is provided for bursty data applications, such as telemetry, train control systems, interactive data access, toll road charging systems, and browsing the World Wide Web (WWW) using the Internet.
A reference model for a GPRS network, as known in the art, is depicted in FIG. 1. GPRS allows a MS 11 to send and receive data in an end-to-end “packet transfer” mode, without using any network resources in “circuit-switched” mode. This allows for autonomous operation of GPRS and best fits the bursty traffic characteristics. Packet routing is supported by definition of a new logical network node called a GPRS support node (GSN). The GSN is basically a packet router with additional mobility management features and connects with various network elements through standardized interfaces. The GSN node that acts as a physical interface to external Public Data Networks (PDN's) 20, such as operator networks, corporate networks, or the Internet, is known as a Gateway GSN (GGSN) 21. The GSN node that connects with a BSC (Base Station Controller) 14 and directly handles packet delivery to and from MS's 11 is known as a Serving GSN (SGSN) 15.
Each SGSN 15 is responsible for the delivery of packets to the MS's 11 within its service area 10. The BSC 14 is the network entity controlling a number of BTS's (Base Transceiver Stations) 13, which are the network entities which communicate with the MS 11.
An SGSN 15 provides a connection point for subscribers when they want to access services provided by the GPRS network 22. The SGSN 15 downloads the capabilities of the connecting MS 11 from a HLR (Home Location Register) 17, along with information such as security, billing and authentication etc. The HLR 17 is a database within the GSM network 22 which stores all the subscriber-specific data.
Before an MS 11 is capable of using a GPRS service, it must attach to an SGSN 15. Effectively, this attachment corresponds to the establishment of a logical link between the MS 11 and its SGSN 15. The SGSN 15 encapsulates the data packets from the MS 11 and routes them to the appropriate GGSN 21, where they are forwarded to a fixed host inside the correct PDN 20. Specific routing policies are applied inside this PDN 20 to send the packets to the corresponding fixed host; these are known as “routing contexts”. After attachment, one or more routing contexts for one or more network protocols can be negotiated with the SGSN 15.
Packets coming from a corresponding fixed host to an MS 11 are first routed to the GGSN 21 through the PDN 20, based on the examination of the destination address. The GGSN 21 checks the routing context associated with this destination address and determines the address of the SGSN 15 currently serving the addressed MS 11. Subsequently, the original data packet is encapsulated into another packet (this procedure is called tunnelling), which is forwarded to the SGSN 15 and ultimately delivered to the correct MS 11.
In order to verify that a given MS 11 is allowed to use a network protocol, the HLR 17 is queried. Among other things, the subscription profile found in the HLR 17 includes the matching GGSN 21 address. If access is permitted, the GGSN 21 is requested to update the routing context (i.e. the SGSN 15 address and tunnelling information) accordingly.
The GPRS backbone network (as defined by links Gn, Gp, and Gi in FIG. 1) is a private IP (Internet Protocol) network. The IP addresses used in this part of the backbone network are selected by the GPRS operator and they are not known outside the backbone network.
Also shown in FIG. 1 is an MSC (Mobile Switching Centre) 16, which is the switching centre of a mobile phone network, a GRX (GPRS Roaming Exchange) 18, which is a point of connection to other GPRS networks, as well as the various links having reference names, as listed below:                A is an interface between the BSC 14 and the MSC 16;        Abis is an interface between the BSC 14 and the BTS 13 (this can be proprietary if the BSC and the BTS are from the same NEM (Network Equipment Manufacturer), which is often the case);        Gb is an interface between the SGSN 15 and the BSC 14, which is likely to be implemented using Frame Relay;        Gi is an interface between the GGSN 21 and the PDNs 20, which is likely to be implemented using IP;        Gn is an interface between the GGSN 21 and the SGSN 15, which is likely to be implemented using IP;        Gp is an interface between the GGSN/SGSN 21, 15 and the GRX 18, which is likely to be implemented using IP;        Gr is an interface between the SGSN 15 and the HLR 17, which is likely to be implemented using SS7 (Signalling System 7); and        Gs is an interface between the SGSN 15 and the MSC 16.        
An example of a GPRS protocol is the GPRS Tunnelling Protocol (GTP), which is a transmission protocol to tunnel multi-protocol packets between GSN's. Several subscribers in the supply area of a single SGSN 15 may be simultaneously connected to a PDN 20 via the same GGSN 21. An IMSI (International Mobile Subscriber Identity) is used to uniquely identify each MS 11 in the GPRS network 22.
An MS 11 may also run several applications simultaneously; each of them requiring connection to a plurality of PDNs 20 connected to the same GGSN 21. Therefore each application must also be uniquely identified. A Network Service Access Point Identifier (NSAPI) is used for this purpose. The NSAPI is assigned when the MS 11 requests a call set-up, a process referred to as the Packet Data Protocol (PDP) Context Activation Procedure. A PDP context (i.e routing context) describes the properties of a link between the MS 11 and the GGSN 21, such as which QoS level is used for the transmission etc.
A transaction, as known in the art, is a related exchange between two network elements, for example a transaction could contain all packets that constitute a “GPRS location update” (which occurs when a mobile subscriber moves from one cell to another), or all the messages to do with a MS 11 attaching to the packet-switched network (such as GPRS Attach Request, Attach complete, Create PDP Context Request, Create PDP Context Complete etc). A transaction builder, also as known in the art, uses knowledge of the protocols used in the network (such as the GPRS protocols) to build a plurality of individual traffic messages into a complete transaction.
Currently operators monitor the status of the network by “grabbing” information from the network elements and then storing or displaying it in a variety of ways, using known network management systems. The main reason for operators and manufacturers to collect data is for reasons such as network performance evaluation. Data, such as statistics concerning how many subscribers are connected to the network at any one time, or how many IP tunnels have been created, can be collected from each network element according to a schedule established by a network management system. Such information is normally maintained in MIB (Management Information Base) tables held inside the network elements themselves and is accessible by network management systems using a network protocol such as SNMP (Simple Network Management Protocol), as known in the art.
Operators, for example, also may want to know where a GPRS mobile user was when they encountered a problem accessing a service or had a network outage, so network managers also build “relationships” i.e. tables of data linking items such as the Cell Location ID (CLI) with a subscriber's IMSI, both of which can be extracted from the GPRS protocols operating at the Gb side of a SGSN (see FIG. 1).
Some operators will not allow access to their network elements, switches and routers etc for such monitoring purposes. For example with some switching devices, network management monitoring, such as that which uses SNMP as previously described, “steals” processing power to produce such statistics and thus increases processing loads and reduces the overall efficiency of the network element. This equates to lost revenue for the operators. Also, firewalls may surround the operator's backbone IP networks and allowing network management systems to have access to these networks, is perceived as reducing the overall security of the system.
Currently the required relationship information can be maintained by state machines. The use of state machines requires that all traffic is observed and introduces significant processing overheads. The current prior art does not generally deal with transactions, but with the individual messages that are then combined into a transaction. The state machines used need to “see” every single message from the interactions between network elements, in order to “follow” the progress of a transaction on the network. There are a number of reasons why this is not a satisfactory solution, one of which is the presence of ciphering on some network element interfaces. Prior art state machine solutions therefore must be able to “grab” cipher key messages and distribute them round the system, so that they can be used to decipher messages on the interfaces being monitored. If keys are missed due to hardware failure or network propagation delays, then it is possible that the prior art implementations will be unable to decipher messages and will subsequently not be able to “feed” them to their state machines.
Prior art transaction builders make use of similar technology and suffer similar problems. If messages cannot be deciphered then any implementation will “miss” transactions, because they cannot be built in the first instance. This infrastructure problem means that any implementation may never be presented with all of the transactions that are present in the network.
It is important, therefore, for a network management system to be able to model the state of telelcommunications network elements, such as, but not limited to, those network elements found in a GPRS network, with a high degree of accuracy, using the information contained within the transactions that can be built from the telecommunications network traffic which is being monitored. Furthermore, it is important that monitoring equipment that can passively tap the traffic passing between network elements (such as those found in a GPRS network) and from information “gleaned” from knowledge of this traffic (for example from knowledge of the GPRS protocols and standard transaction formats), can build and maintain the required significant relationships and can model the internal state of network elements (such as GPRS network elements).
In particular, it is important for a network manager to be able to obtain the same results that would previously have needed direct access to an operator's network elements, but without the need for implementing a state machine based solution, using deciphering keys, or by having direct access to those network elements.
The present invention seeks to solve, or at least mitigate, the problems of the prior art by providing a method and apparatus for monitoring a telecommunication network having a plurality of users and network elements, especially, though not exclusively, where direct access to those network elements is not available.